Xerographic charging apparatus with means to terminate the charging cycle when a predetermined charge is obtained



Aug. 8, 1967 E -w up 3,335,273

XEROGRAPHIC CHARGING APPARATUS WITH MEANS TO I TERMINATE THE CHARGINGCYCLE WHEN A PREDETERMINED CHARGE IS OBTAINED Filed Dec. 4, 1964 2Sheets-Sheet l CONTROL CIRCUIT 6 0. 0. POWER SOURCE r m4 mam if cmcu/rF/ G. I L84 IN VEN TOR LEWIS E. WALKUP /V A T TORNEVS Aug. 8, 1967 1..E. WALKUP 3,335,273

XEROGRAPHIC CHARGING APPARATUS WITH MEANS TO TERMINATE THE CHARGINGCYCLE WHEN A PREDETERMINED CHARGE IS OBTAINED 2 Sheets-Sheet 2 FiledDec. 4, 1964 INVENTOR LEWIS E.WALKUP y f/f m ATTORNEYS United StatesPatent 3,335,273 XEROGRAPHIC CHARGING APPARATUS WITH MEANS T0 TERMINATETHE CHARGING CY- CLE WHEN A PREDETERMINED CHARGE IS OBTAINED Lewis E.Wallrup, Columbus, Ohio, assignor, by mesne assignments, to XeroxCorporation, Rochester, N.Y., a corporation of New York Filed Dec. 4,1964, Ser. No. 415,966 3 Claims. (Cl. 250-495) ABSTRACT OF THEDISCLOSURE A circuit for charging an electrostatic plate having acapacitor connected in series with a plate and a control circuitincluding a thyratron device for controlling the charging current to theplate upon variations of the voltage drop developed across the capacitorand the plate.

This invention relates to the field of Xerography and, particularly, toan improved electric circuit to control a corona generating device forapplying electrostatic charge on a xerographic plate.

By present techniques, the charging of an in-place xerographic plate inpreparation for the exposure step is accomplished by means of a coronagenerating device whereby an electrostatic charge on the order of 500 to600 volts is applied to the xerographic plate. A form of coronagenerating device for this purpose may comprise a plurality of parallelwires connected in series to a high voltage source and supported in aconductive shield that is arranged in closely spaced relation to thesurface to be charged. When the wires are energized, corona is generatedalong the surface of the wire and ions are caused to be deposited on theadjacent photoconductive surface.

As is well known, variations in the potential applied to corona wires ofa given diameter will cause relatively large changes in corona currentwith corresponding variations in the charging rate. In addition, thecorona threshold potential and corona current are also affected directlyby deposits of dust that may accumulate on the wire, by atmosphericconditions such as humidity, temperature and pressure. Thus whenoperating at the corona threshold, variations in voltage supply, slightaccumulations of dust on the wire, and variations in air current andatmospheric conditions drastically affect the corona generatingpotential of the wire and cause a non-uniform electrostatic charge to bedeposited on the xerographic plate.

In the art of xerography it has been established that consistent highquality reproductions can best be effected when a uniform potential isapplied to a xerographic plate to prepare the plate for the exposurestep. If the xerographic plate is not charged to a sufiicient potential,the electrostatic latent image obtained upon exposure will be relativelyweak and the resulting deposition of a developer material thereon willbe correspondingly small and, if the xerographic plate is overcharged,the converse will occur, and, if overcharged sufliciently, thephotoconductive layer of the xerographic plate may be permanentlydamaged.

Since the contrast value, comparable to the contrast value obtainablefrom silver halide papers, of the electrostatic latent image is relateddirectly to the potential charge on the xerographic plate beforeexposure, it is apparent that if the plate is not uniformly charged overits entire area, the contrast value of the electrostatic latent imageobtained upon exposure will vary in different areas on the plate, and astreaky effect will be visible on the image when developed.

It is, therefore, the principal object of this invention to improve theelectrical circuit of a corona generating 3,335,273 Patented Aug. 8,1967 device whereby a uniform electrostatic charge may be deposited on axerographic plate.

A further object of this invention is to improve a corona generatingdevice control circuit for use in xerographic machines wherein it isdesirable to charge a Xerographic plate to a uniform potentialregardless of variations in the supply line voltage or changes inthe-surrounding atmospheric conditions.

These and other objects of the invention are attained by cooperativeaction with a corona generating device comprising a mounting frame andcoronode wires mounted and extending within the frame to charge axerographic plate by corona discharge. The novel charging circuit forensuring a constant charging current includes a capacitor connected inseries with a xerographic plate and a ground circuit arrangement forcontrolling the charging current in accordance with variations of thevoltage drop developed across the capacitor and the xerographic plate.

For a better understanding of the invention as well as other objects andfeatures thereof, reference is had to the following detailed descriptionof the invention to be read in connection with the accompanyingdrawings, wherein:

FIG. 1 illustrates schematically a preferred arrangement of axerographic plate, a corona generating apparatus and circuit therefor inaccordance with the invention; and,

FIG. 2 is a schematic electrical wiring diagram of the control circuitfor the corona discharge device of FIG. 1.

Referring now to the drawings, there is disclosed a preferredarrangement of a corona generating device of the invention as applied toa xerographic plate. As shown in FIGS. 1 and 2, the present invention isadapted to be applied to a xerographic plate having a photocoductivelayer or radiation-receiving surface 10, such as selenium, on aconductive backing plate 12.

For the purpose of the present disclosure, the several xerographicprocessing stations for practicing xerography may be describedfunctionally, as follows:

A charging station, at which a uniform electrostatic charge is depositedon the photoconductive layer of the xerographic plate;

An exposure station, at which a light or radiation pattern of copy to bereproduced is projected onto the plate surface to dissipate the platecharge in the exposed areas thereof and thereby form a latentelectrostatic image of the copy to be reproduced;

A developing station, at which a xerographic developing materialincluding toner particles having an electrostatic charge opposite tothat of the electrostatic latent image is cascaded over the platesurface, whereby the toner particles adhere to the electrostatic latentimage to form a xerographic powder image in the configuration of thecopy to be reproduced;

A transfer station, at which the xerographic powder image iselectrostatically transferred from the plate surface to a transfermaterial or support surface;

A plate cleaning and discharge station, at which the plate surface isbrushed to remove residual toner particles remaining thereon after imagetransfer, and at which the plate surface is exposed to a relativelybright light source to effect substantiallycomplete discharge of anyresidual electrostatic charge remaining thereon; and

A fixing station whereat the powder image on the support surface isfused or permanently fixed to the surface.

Referring now to the subject matter of the invention and, particularlyFIG. 1, the electrostatic charging of the xerographic plate inpreparation for an exposure step is accomplished by means of a coronagenerating device whereby an electrostatic charge is applied to theplate surface as it is positioned adjacent the charging device. The

potential applied to the plate is dependent upon the particular printcontrast desired whereby higher print contrasts require higher initialplate potentials. The charging device comprises a fiat frame structure14 having side members 16, 18 spaced apart by transverse members 20, 22which connect the ends of the side members, respectively, to form aframe structure.

To efiect charging of the plate there is provided a corona generatingdevice and a charging circuit to sup ply electrical power to the coronagenerating device. The corona generating device shown in FIG. 2 includeswires 54 called the coronode, which, by a corona discharge, charge thephotoconductive surface of the Xerographic plate 10, 12. The potentialapplied to the plate surface can be controlled by a circuit to bedescribed hereinafter. The wires 54 are made of any suitablenon-corrosive material, such as stainless steel, having a uniformexterior. In a preferred embodiment of the charging device, the diameterof the wires is approximately three-and-one-half thousandths of an inch,it being apparent that other size.

wire can be used.

The corona charging circuit of FIG. 2 is provided with a DC). powersource in the form of a rectifier circuit generally indicated by thereference numeral 60. The rectifier circuit is connected by a pair ofconductors 62, 63 to a source 64 of alternating current, such as acommercial outlet of 110 volts and is adapted to produce a DC. potentialin the range of approximately 6,000 to 11,000 volts. A start switch 65is provided in the conductor 62 for connecting the various circuits tothe source 64. The conductors 62,63 are connected to the primary windingTP-l of a high voltage transformer having its secondary TS-l as theelectrical supply for the rectifier circuit 60.

Connected in series in the conductor 63 is a control circuit generallyindicated at 66 which serves to control voltage impressed upon theprimary winding TF4. The circuit 66 comprises a pair of triodes 68, 69each of which has its anode connected to the cathode of the other withthe connection points thereof being connected to the conductor 63. Asecondary transformer winding TS-2 ,is connected between the grid andthe cathode of triode 68 and a secondary transformer TS-3 is, connectedbetween the grid and cathode of the triode 69. The primary winding TP-2for the secondary windings TS-2 and TS-3 is connected at one end to thepower conductor 62 and at its other end to a wiper arm 70 of apotentiometer resistor 72 which is connected between the supplyconductors 62,63. The winding TP-2 is associated with the secondarywindings TS-2 and TS-3 for supplying the tubes 68, 69 with sufficientbias to control conduction of these and, therefore, the energization ofthe primary TP-l.

The wiper arm 70 of the potentiometer is linked toa pressure-sensitizedbellows 74 which serves to control positioning of the arm along theresistor 72in accordance with ambient pressures. This arrangementautomat- 1 ically regulates the plate-charging process during changes inambient pressure by regulating the corona voltage within a safeoperating range for any ambient pressure.

In the rectifier circuit 60, the secondary winding TS-l is connected atone end to the cathode of a first diode 76 and at its other end to theplate of a second diode 78. The

"plate of the first diode is connected to the cathode of the seconddiode and to ground to maintain these elements at ground potential. Thetwo ends of the winding TS-1 are also connected to the corona wires 54for the corona generating device 18. With this arrangement there is aDC. potential continually supplied to the corona wires 54 forsensitizing charging the xer-ographic plate 10, 12.

A capacitor 80 is electrically connected on one side to Xerographicplate 10, 12 and, a voltage divider comprising two resistors 82, 83 inseries is connected across this capacitor. The resistors 82, 83 formpart of a thyratron control circuit, generally indicated at 84. Thejunctionpoint for the resistors is connected to the control grid 85 of athyratron 86 having its cathode 88 through a resistor 89 connected tothe other side of the capacitor 80 and to tubes ground. Both thecathode. and this side of the capacitor are maintained at groundpotential. The AC. plate sup ply for the thyratron is derived from theconductor 63 by way of a conductor 90 connected to the plate 92 of thethyratron.

A relay 94 is connected in the conductor 90 in series with the plate 92and is adapted to become energized during conduction of the thyratronfor opening a normally closed relay contact switch 96 connected inseries with the primary winding TP1. A normally open contact switch 98is also associated with the relay 94 and is connected in series with amanually actuable reset switch 100 and between the junction point forthe resistors 82, 83 and the cathode 88. When the relay 94 is energized,the switches 96, 98 are actuated to open and closed conditions,respectively, and, conversely, when the relay is in a dcenergized state,the switch 96 will be closed and the switch 98 opened.

For control of the charging current on the corona wires 54, thecapacitance of the capacitor between the plate 10, 12 and ground is maderelatively large compared to the capacitance through the selenium layer10; for example, a suitable relative ratio between these capacities maybe 20 to 1. With this relationship, as the corona charge accumulates onthe selenium layer 10, the potentialtherebetween is raised because ofthe potential drop through the selenium layer and the potential dropacross the capacitor 80. When the sum of these two potentials reaches asufficiently high value during a plate charging cycle which will beindicative of a predetermined charge upon the plate, say, for example,600 volts, corona charging of the plate will be controlled in order tomaintain the predetermined level between the plate and the corona Wires54. With this arrangement, complete control is possible over the valueof the potential on the selenium layer and, consequently, the amount ofcorona on the layer by utilizing the voltage drop across the capacitor.

To illustrate, it will be assumed that 600 volts is necessary on theXerographic plate and that 7,000 is to be used as a charging potential.The DC. corona potential on the surface of the plate with respect toground will reach 3,000 at the time when the potential drop across theselenium reaches 600 volts. In order to accomplish 600 volts across theselenium layer and with the capacitance of the capacitor 80 being 20times greater than that of the selenium layer, the potential drop acrossthe capacitor 80 must be 30 volts, and this drop will be utilized tocontrol or maintain the charge of 600 volts on the plate. It will beapparent that by changing the capacity of the capacitor 80, otherpotentials on the selenium layer may be produced.

The voltage across the capacitor controls closely the larger voltage onthe xerographic plate through the operation of the thyratron. When thevoltage drop across the capacitor 80 reaches a predetermined value,depending upon the desired voltage across the plate and the.

capacitance of the capacitor, a control voltage, as determined by thesevalues and the action of the voltage divider 82, 83, is fed to thecontrol grid of the thyratron. When at a predetermined value, thecontrol voltage will produce conduction in the thyratron which actionwill energize switch 96 to open the primary circuit to the high voltagetransformer, thereby terminating charging of the xerographic plate.

Specifically, the voltage divider 82, 83 as applied to the capacitor 80and to the voltage drop thereacross results in a negative voltage beingapplied to the grid 85 and, consequently, the prevention of conductionin the thyratron. As the voltage drop across the capacitor increaseswith the charging of the Xerographic plate, this voltage will approachzero and, when at a predetermined level due to full charging of theplate, at the time that the plate 92 open the circuit that producescharging of the plate.

When the thyratron ceases conduction during the negative half cycle ofthe supply voltage, the charging will be restored until the nextpositive half cycle. In effect then, the signal on the grid controls theperiod during each cycle that the thyratron may conduct during chargingof the xerographic plate and for maintaining a charge thereon. Duringthe charging cycle, the flow of ions to the plate 10, 12 is continuouslymaintained under control of the thyratron circuit. After the charginghas terminated, the start switch 65 is opened and the plate 10, 12removed whereupon the reset switch 100 is momentarily closed thuscausing the grounding and discharging of the capacitor 80 which, ineffect, dissipates the bias on the grid 85. With this condition of thethyratron, the relay 94 will become deenergized causing the opening ofthe switch 98 and the closing of the switch 96 to condition the coronacharging device and the control circuit for another charging cycle.

While the invention has been described with reference to the circuitdisclosed herein, it is not confined to the details set forth since itis apparent that certain electrical equivalent components may besubstituted for the components of the preferred circuit withoutdeparting from the scope of the invention. This application is thereforeintended to cover such modifications or changes as may come within thepurposes of the invention as defined by the following claims.

What is claimed is:

1. An appaartus for charging a Xerographic plate including a coronagenerating device having at least one corona discharge wire positionedadjacent to a Xerographic plate to impose an electrostatic charge ontothe Xerographic plate; a source of alternating current; a high voltagedirect current supply electrically connected to said source and beingelectrically coupled to said corona discharge wire for charging thesame; a control tube having a cathode electrically connected to ground,an anode and a grid, a relay electrically connected in series with saidanode and being energizable during conduction of said tube; said relayincluding a normally closed contact switch interposed between saiddirect current supply and said source; a capacitor electricallyconnected between the xerographic plate and said cathode and beingelectrically coupled to the grid of said control tube for imposing abias on said grid in accordance with the charging current through thexerographic plate, said capacitor having a capacitance whereby avariation in the charging current above a predetermined value will varythe grid bias to initiate conduction of said tube for energizing saidrelay thereby opening the circuit between the source and the dischargewire.

2. An apparatus for charging a xerographic plate including a coronagenerating device having at least one corona discharge wire positionedadjacent to a xerographic plate to impose an electrostatic charge ontothe Xerographic plate; a source of alternating current; a high voltagedirect current supply electrically connected to said source and beingelectrically coupled to said corona discharge wire for charging thesame; a pressure sensitive device mechanically connected between saidsource and said supply for varying the alternating current to saidsupply in accordance with atmospheric pressure; a thyratron having acathode electrically connected to ground, a first grid and an anodeconnected to said source and, a second grid; a relay connected in serieswith said anode and being energizable during conduction of saidthyratron; said relay including a normally closed contact switchinterposed between said direct current supply and said source; acapacitor electrically connected between the xerographic plate and saidcathode and being electrically coupled to the second grid of saidthyratron for imposing a bias on that grid in accordance with thecharging current through the Xerographic plate, said capacitor having acapacitance whereby a variation in the charging current above apredetermined value will vary the second grid bias to initiateconduction of said tube for energizing said relay thereby opening thecircuit between the source and the discharge Wire.

3. An apparatus for charging a Xerographic plate including a coronagenerating device having at least one corona discharge wire positionedadjacent to a xerographic plate to impose an electrostatic charge ontothe Xerographic plate; a source of alternating current; a high voltagedirect current supply electrically connected to said source and beingelectrically coupled to said corona discharge wire for charging thesame; a control tube having a cathode electrically connected to ground,an anode and a grid; a relay electrically connected in series with saidanode and being energizable during conduction of said tube; said relayincluding a normally closed contact switch interposed between saiddirect current supply and said source; a capacitor electricallyconnected between the xerographic plate and said cathode and beingelectrically coupled to the grid of said control tube for imposing abias on said grid in accordance with the charging current through theXerographic plate, said capacitor having a capacitance whereby avariation in the charging current above a predetermined value will varythe grid bias to initiate conduction of said tube for energizing saidrelay thereby opening the circuit between the source and the dischargewire; and means for selectively discharging said capacitor after theXerographic plate has attained said predetermined level of charge.

References Cited UNITED STATES PATENTS 3,062,956 11/1962 Godichini250-49.5

RALPH G. NILSON, Primary Examiner. W. F. LINDQUIST, Assistant Examiner.

1. AN APPARATUS FOR CHARGING A XEROGRAPHIC PLATE INCLUDING A CORONAGENERATING DEVICE HAVING AT LEAST ONE CORONA DISCHARGE WIRE POSITIONEDADJACENT TO A XEROGRAPHIC PLATE TO IMPOSE AN ELECTROSTATIC CHARGE ONTOTHE XEROGRAPHIC PLATE; A SOURCE OF ALTERNATING CURRENT; A HIGH VOLTAGEDIRECT CURRENT SUPPLY ELECTRICALLY CONNECTED TO SAID SOURCE AND BEINGELECTRICALLY COUPLED TO SAID CORONA DISCHARGE WIRE FOR CHARGING THESAME; A CONTROL TUBE HAVING A CATHODE ELECTRICALLY CONNECTED TO GROUND,AN ANODE AND A GRID, A RELAY ELECTRICALLY CONNECTED IN SERIES WITH SAIDANODE AND BEING ENERGIZABLE DURING CONDUCTION OF SAID TUBE; SAID RELAYINCLUDING A NORMALLY CLOSED CONTACT SWITCH INTERPOSED BETWEEN SAIDDIRECT CURRENT SUPPLY AND SAID SOURCE; A CAPACITOR ELECTRICALLYCONNECTED BETWEEN THE XEROGRAPHIC PLATE AND SAID CATHODE AND BEINGELECTRICALLY COUPLED TO THE GRID OF SAID CONTROL TUBE FOR IMPOSING ABIAS ON SAID GRID IN ACCORDANCE WITH THE CHARGING CURRENT THROUGH THEXEROGRAPHIC PLATE, SAID CAPACITOR HAVING A CAPACITANCE WHEREBY AVARIATION IN THE CHARGING CURRENT ABOVE A PREDETERMINED VALUE WILL VARYTHE GRID BIAS TO INITIATE CONDUCTION OF SAID TUBE FOR ENERGIZING SAIDRELAY THEREBY OPENING THE CIRCUIT BETWEEN THE SOURCE AND THE DISCHARGEWIRE.